Method and apparatus for measuring structures in a fingerprint

ABSTRACT

A plurality of images of portions of a fingerprint surface is generated by measuring structural features the portions of the surface with a sensor array as the surface is moved relative to the array. A two-dimensional image of the fingerprint surface is constructed from a portion of the plurality of images. In one embodiment, a varying voltage is applied to a finger positioned over an exciting electrode and a capacitive sensor array, and the capacitance or impedance through the finger is measured between the electrode and the array to detect variations in capacitance or impedance caused by variations in the structural features of the fingerprint surface. In one embodiment, the speed of the fingerprint surface relative to the sensor array is determined by sensing features of the fingerprint surface at two spaced-apart sensing elements and determining the speed from the distance between the sensing elements and the time lapse between passage of identical features of the fingerprint surface from one of the sensing elements to the other.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 09/424,210 filed Nov. 22, 1999 (now U.S. Pat. No. ______), which isa 371 of PCT/N098/00182 filed Jun. 12, 1998, which claims priority fromNorwegian Application No. 972759 filed Jun. 16, 1997, the disclosures ofwhich are hereby incorporated by reference.

BACKGROUND

1. Field of the Invention

The invention relates to a method and an apparatus for the measuring ofstructures in a fingerprint or the like, comprising the measuring ofchosen characteristics of the surface of the fingerprint, e.g.,capacitance or resistivity, using a sensor array comprising a pluralityof sensors, positioned in contact with, or close to, the surface.

2. Description of the Related Art

Identification by the use of fingerprints has lately come to the fore asa result of the increasing needs for security relating to, for example,credit cards or computer systems, as well as the greatly increasedavailability of pattern recognition algorithms. Some systems forrecognition of fingerprints have already been made available on themarket. The techniques used to register the fingerprint varies.

Some of the previously known solutions are based upon optical technologyusing light with one or more wavelengths. These are sensitive to dirtand contamination, both in the fingerprint and on the sensor surface,and thus cleaning is necessary for both.

Another alternative is pressure measurement, such as is described inU.S. Pat. No. 5,559,504, U.S. Pat. No. 5,503,029 and U.S. Pat. No.4,394,773. This, however, has the disadvantage that the sensor surfacebecomes sensitive to mechanical wear and damage, as the sensor has tohave an at least partially compliant surface.

Temperature sensors have also been suggested, for example in U.S. Pat.No. 4,429,413 and international patent application PCT/NO96/00082.

Since fingerprint sensors may be exposed to long term use in varying andsometimes demanding conditions the sensor needs to have a robust surfaceand to be as insensitive to pollution in the fingerprint and on thesensor as possible. It must be capable of reading most fingerprintswithout being disturbed by latent prints from earlier use. In somecases, e.g., in credit cards or computer keyboards, it would also beadvantageous if the sensor could be made compact.

In the view of costs there is also a demand for simplicity andminimizing of the number of parts.

In addition to the solutions mentioned above, the measuring ofcapacitance has been tried as a method to measure fingerprints. Examplesare shown in U.S. Pat. No. 4,353,056 and U.S. Pat. No. 5,325,442. Whilethe ridges of the fingerprint touches the sensor surface, the valleyshave a small distance to the sensor surface, resulting in a differencein capacitance and/or conduction measured at the different sensors.Humidity may affect the measurements, but if it is even throughout thefingerprint an analysis of the contrast between the measurements canprovide a picture of the fingerprint.

All the solutions mentioned above are based upon two-dimensional sensorarrays with dimensions comparable to the size of the fingerprint. Theseare expensive and difficult to produce, since they comprise a largenumber of sensors simultaneously measuring the surface.

EP 735,502 describes the use of a one or two-dimensional array ofsensors being moved in relation to the fingerprint. The describedsolution is based on the measuring of resistance, and has a limitedresolution defined by the minimum sensor dimensions and the distancebetween the sensors.

SUMMARY

It is an object of the present invention to provide a sensor being easyto produce, making them cheap in production, and also relatively small.

The present invention provides a method and an apparatus for themeasuring of structures in a fingerprint or the like, for example usingone of the techniques described above, characterized as stated in thedisclosed claims.

As the surface of the sensor array is small, and contains few sensorscompared to the known solutions, it is inexpensive and relatively simpleto make. As the fingerprint to be measured is moved past the sensorarray there is no latent fingerprint remaining from the previous user,giving another advantage in relation to the known fingerprint sensors.

Since the details in the fingerprints are small, it is also difficult tomake the sensors of the detector small enough. In a preferred embodimentthe apparatus and method according to the invention comprises two ormore parallel lines of measuring points, each line of measuring pointsbeing shifted in the longitudinal direction with a distance less thanthe distance between the measuring points, the sensor array comprisingtwo or more parallel lines of equally spaced sensors, preferably shiftedin the longitudinal direction of the sensor array. This provides apossibility to measure structures in the fingerprint smaller than thespacing of the sensors. This is not possible with any of the previouslyknown detector systems.

Thus, it is to be understood that the term “essentially one-dimensionalarray” here refers to an array having a length being much larger thanits width, and may comprise more than one line of sensors.

The invention will be described below with reference to the encloseddrawings, which illustrate one possible embodiment of the invention.

DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show schematic views of two versions of the sensor.

FIG. 2A illustrates the sensor in FIG. 1B in use, as seen from above.

FIG. 2B shows a cross section of the situation in FIG. 2A.

FIG. 3 shows a schematic view of an apparatus according to theinvention.

FIG. 4 shows a cross section of an embodiment of the invention.

FIG. 5 shows a preferred embodiment of the invention.

DETAILED DESCRIPTION

In FIG. 1A a single, linear array of sensors 1 is shown. The sensors maybe of different kinds, such as pressure sensors or temperature sensors,but preferably they are electrical conductors able to measureconduction, impedance or capacitance of the different parts of thefingerprint. The surface to be measured is moved in a perpendiculardirection relative to the line of sensors.

In the preferred embodiment, the sensors 1 are electrical conductorsseparated by an insulating material 3, such as epoxy. In the shownembodiment an electrically conducting material 2 surrounds the sensorswhich may be used to provide a reference potential. Thus, theconduction, impedance, or capacitance through the fingerprint betweeneach of the sensors 1 and the surrounding reference level may bemeasured.

The shown embodiment having equally spaced sensors is preferred, butother solutions, e.g., comprising groups of sensors for measuringcertain parts of the fingerprint, are also possible.

Using one or more sensors positioned at one or more chosen distancesfrom the sensor line will enable measuring the velocity of thefingerprint in relation to the sensor by comparing the signals from thesensor line and the time lapse or spatial shift between the measurementsof corresponding structures in the surface. FIG. 1B shows a preferredembodiment of the invention in which the sensor array comprises twolines of sensors 1.

To be able to measure the structures in a fingerprint the array willtypically be 10 15 mm long with a resolution of 50 μm. This is difficultor expensive to obtain using a single line of sensors. In FIG. 1B thelines are slightly shifted in relation to each other. When moving asurface across the sensor array, the measurements of each of the sensorsin the second line will fall between the measured point of the firstline, providing the required resolution with a larger distance betweenthe sensors. Three or more lines are possible to improve the resolutioneven more, but more than five would be impractical because of thedistance between the lines and the resulting time lapse between themeasurements of the first and the last line. Also, an apparatus usingmany lines would be sensitive to the direction in which the finger ismoved.

Although the lines shown in the drawings comprise equally spacedsensors, the shifted second, third, etc. lines may comprise single orgroups of sensors, increasing the resolution in certain parts of thefingerprint, and/or measuring differences in velocity of different partsof the fingerprint, in case the movements are uneven. Also, the second,third, etc. lines may have an angle in relation to the first line ofsensors.

When using a sensor array comprising two or more sensor lines as shownin FIG. 1B, the measurements of the different lines must be combined toprovide a signal corresponding to one single line of sensors. To do thisthe signals from the sensors must be adjusted for the time delay betweenthe signals from the sensors in different lines, and thus the speed ofmovement of the finger in relation to the sensor array must be known,either by moving the finger or sensor array with a chosen speed, or bymeasuring the speed of the finger.

FIG. 2A illustrates how the finger 4 is moved over a sensor array in thedirection perpendicular to the array. In order to obtain exactmeasurements the movement of the finger must be measured. In addition tothe above-mentioned method comprising the correlation of measurementsfrom different sensors, this may be done in many ways such as providinga rotating cylinder in contact with the finger so that the rotation ofthe cylinder may be measured. Another example may be the use of a thindisk on which the finger may be positioned which is moved together withthe finger and is connected to the apparatus so that the velocity of thedisk may be measured. Preferably, however, the movement is measured bycorrelating or comparing the signals from the different sensor lines,and the time lapse or spacial shift between the measurements ofcorresponding structures in the surface is found. This way more detailedimages can be made from the separate images of each line of sensors.

Another method for adjusting for the movement of the finger is tomaintain the sampling rate at the sensor array while adjusting thenumber of measured lines used in generating the segmented image of thesurface. Thus, the interval of the measurements is adjusted according tothe speed in order to obtain at least one measurement of each portion ofthe surface. For example, if the fingerprint is moved slowly over thesensor while the sampling or measuring frequency is high, the redundantdata may simply be neglected and the image of the fingerprint iscomprised by each second or third set of data.

FIG. 2B shows a cross section of the finger 4 placed on the sensors 1and also shows an exaggerated view of the ridges 5 and valleys 6 in thefingerprint.

FIG. 3 shows a simplified view of the apparatus according to theinvention comprising conductors from the sensors 1 to an amplifier andmultiplexer 8. The signal is then digitized in an A/D-converter 9 beforethe digital signal is sent to a computer 10 comprising any availablecomputer program being able to analyze the signal.

A cross section of a more realistic embodiment is shown in FIG. 4 inwhich one end of each of the closely spaced conductors 11 represents thesensors and the other end of these conductors is connected to amicrochip 15. The conductors 11 may be a part of a multilayer printedcircuit board moulded in epoxy, producing two or more lines of sensors.Each sensor 1 would be about 35×50 μm. If the sensors in each line aremounted with distance between the centres of 150 μm, the resolution withthree shifted lines will be 50 μm.

FIG. 5 shows an embodiment of the invention where an external timevarying, e.g., oscillating or pulsating, voltage 12 is applied to thefinger through the conducting area 14 on the side of the sensor area.Planes at a constant voltage 13 are placed close to and parallel to thelines of sensors 1 This reduces cross-talk and noise from externalsources, and improves contrast in the image generated from themeasurements. This may be implemented by using a multilayer printedcircuit board, where one or more of the conducting layers are at aconstant voltage. An insulating layer (not shown) preferably covers theconductors 1, 11, and shielding planes 13. The conducting area 14 mayalso be covered by an insulating layer but this would decrease thesignal strength. For better performance, the oscillating voltage 12 maybe applied to both sides of the sensor surface. The oscillating voltagemay, as mentioned above, be a pulse train or a sinus.

In one embodiment a sinus of 100 kHz is applied to the conducting area14, and each of the conductors 11 is terminated by a resistance, and thesignal is amplified and fed to a demodulator, multiplexer, andanalogue-to-digital converter. One advantage of this embodiment is thatthere is essentially no signal on the conductors 11 in the sensor areawhen no finger is present, thus reducing problems with offset voltagesvarying with time and drift in the electronics.

This solution provides a sensor apparatus being simple to produce usingstandard techniques and thus cheap. It is also compact and rugged. Ifthe measured parameter is the resistance, the sensors being the ends ofthe conductors will not change their characteristics as they and thesurrounding epoxy are worn down. If the capacitance is to be measured, adurable insulating layer is provided on the sensors or conductor ends.

The preferred layout of the sensor also allows the resolution to bebetter than the distance between the sensors, reducing cross-talkbetween the sensors.

The method and apparatus according to the invention may, of course, beutilized in many different ways and different characteristics may bemeasured in order to provide a representation of the measured surface inaddition to capacitance and/or conductivity. Optical detectors may beused, and preferably transmitters, so that the reflected image of thefingerprint may be analyzed regarding for example contrast and/orcolour.

The sensors may, as mentioned above, simply be the ends of conductorsconnected to means for measuring capacitance and/or conductivity or maybe sensors made from semi-conducting materials. A preferredsemiconducting material when cost is essential would be silicon.

In the embodiment comprising capacitance measurements an insulatinglayer (not shown) is provided between the conductor ends and thefingerprint.

Another possible embodiment within the scope of this invention comprisessensor lines of not equally spaced sensors positioned to measure chosenparts of the fingerprint.

1. An apparatus for sensing a fingerprint comprising: an essentiallyone-dimensional sensor array including a plurality of sensors andassociated circuitry constructed and arranged to generate a signal ateach of a plurality of different portions of a fingerprint surface atpredetermined intervals of time as the fingerprint surface is movedrelative to said sensor array in a direction that is generallyperpendicular to said sensor array, each signal corresponding to surfacefeatures of the associated portion of the fingerprint surface; and atleast one external electrode positioned adjacent said sensor array andcoupled to said circuitry, said circuitry being adapted to apply avoltage to said external electrode relative to said sensors.
 2. Theapparatus of claim 1, wherein said external electrode is parallel tosaid sensor array.
 3. The apparatus of claim 1, wherein each signal isdependent on impedance measured at said sensors.
 4. The apparatus ofclaim 1, wherein each signal is generated based on at least one ofresistance, conductivity, and capacitance between said externalelectrode and each of said sensors
 5. The apparatus of claim 1, whereinsaid sensors are arranged in a single line.
 6. The apparatus of claim 1,wherein said sensors are arranged in two or more lines, each of saidlines being generally parallel to each other.
 7. The apparatus of claim6, wherein the sensors within each line are equally spaced from oneanother and said two or more lines are spaced apart from one another bya distance that is different from a distance between adjacent sensorswithin each line.
 8. The apparatus of claim 6, wherein the sensors ofone line are shifted longitudinally with respect to the sensors of anadjacent line.
 9. The apparatus of claim 1, wherein said externalelectrode is positioned such that at least a portion of the voltageapplied to said external electrode will travel through the finger to thesensors as the fingerprint surface is moved relative to said sensorarray.
 10. The apparatus of claim 1, wherein said circuitry is adaptedto apply a varying voltage to said external electrode relative to saidsensors.
 11. The apparatus of claim 1, wherein said circuitry comprises:an amplifier and multiplexer connected to said sensors by conductors; anA/D converter adapted to digitize a signal from said amplifier andmultiplexer; and a computer programmed to analyzed the digitized signal.12. The apparatus of claim 1, further comprising planes of constantvoltage positioned adjacent to said sensor array.
 13. The apparatus ofclaim 12, wherein said planes of constant voltage are parallel to saidsensor array.